Two basic types of low-voltage transformers are typically used in lighting systems: magnetic transformers and electronic transformers. Magnetic transformers tend to be large, expensive, and heavy, so their electronic alternatives are often used instead. Electronic transformers receive a standard 60 Hz AC mains (or similar) voltage and, using an oscillator circuit that drives a solid-state transformer circuit, step down the input AC input voltage to a lower-voltage, high-frequency (usually above 20 kHz) AC output. The electronic transformer may be tapped such that an auxiliary winding of the transformer picks up magnetic flux generated by the output current to the transformer's load. This self-feedback is used to keep the transformer oscillating. If the output current is not high enough, however, the oscillation may become unstable or stop completely.
Currently, light-emitting diode (“LED”) replacement lamps use off-the-shelf LED-driver integrated circuits (“ICs”) designed to drive the one or more LEDs in the lamp at a constant current. As the input voltage falls, therefore, the IC automatically pulls more input current from the input supply to try to maintain a constant output power and current delivered to the LEDs. Conversely, when the input voltage rises, the LED driver pulls less input current from the input supply, which again stabilizes the current and power delivered to the LEDs. Some LED drivers include an input-current limiting circuit to protect input components if an undervoltage condition occurs on the input; an LED driver without such a limiting circuit might attempt to draw a damaging amount of input current to compensate for the low input voltage. Other LED drivers may use an undervoltage-lockout (“UVLO”) circuit to achieve the same protection.
While this circuit configuration may work for some lighting applications, it may cause problems in combination with electronic low-voltage transformers of the type commonly used in LED lighting systems. The feedback mechanism in the electronic transformer responsible for keeping the transformer oscillating is driven only by the current drawn from the transformer. As the voltage waveform output by the transformer and applied to the LED driver rises, a prior-art LED driver pulls less current. This reduction in current drawn from the electronic transformer may cause it to shut down if the current draw falls below what is required (i.e., the hold current of the transformer). On the other hand, if the input voltage drops too low, the LED driver pulls higher current, thereby unnecessarily increasing the power delivered to the LED lamp. Because the LED lamp can dissipate only a limited amount of power, this increase in power delivered may result in an over-temperature condition in the LED lamp. A need therefore exists for a way to prevent an LED driver from drawing both too little or too much current from an electronic transformer.